Symmetry and self-assembly in vertebrate A-filaments

The myosin-containing A-filaments of vertebrate skeletal muscle contain 294 myosin molecules packed to give overall 3-fold rotational symmetry, as illustrated by the fraying of the filament into 3 sub-filaments ( Maw and Rowe, 1980). Further studies on slightly frayed filaments are consistent with a highly linear arrangement of these sub-filaments, at least in the major part of the cross-bridge region where sub-filaments can be observed. Isolated filaments have an unusual hydrodynamic property in the form of an anomalous frictional increment. This property is as yet unexplained; it may possibly be related to flow-induced cyclic movements in the myosin heads. Self-assembly of A-filaments in vitro to correct length and width has yet to be achieved. We have found however that under certain exactly defined conditions a very accurate reconstruction of both filaments and A-band can be accomplished in situ. Solubilisation of the myosin in chloride-free medium and maintenance of a high local myosin concentration are absolute requirements. Reconstruction is either abolished or modified by preglycerolation or at longer sarcomere length. It is argued that these results suggest a role for the actin filament lattice in myosin assembly, and imply that myosin assembly in the M-line region may be separable from myosin assembly in the cross-bridge region.